The present invention relates to a heating device for heating fluid, such as air, and more particularly, to an improvement in the arrangement of the heating device employing, for the source of heat, ceramics, such as a thermistor having a positive temperature coefficient characteristic.
One conventional heating device is shown in FIG. 1 in which a ceramic block 1 having a plurality of through-holes 2 formed in the direction parallel to the direction of thickness of the ceramic block 1 is disposed in the path of flow of air generated by the fan 5. The ceramic block 1 has first and second electrodes 3 and 4 on the opposite flat surfaces thereof except on the openings of the through-holes 2. When the voltage is applied between the electrodes 3 and 4, an electric current flows through the ceramic block 1 in the direction of thickness thereof and, as a result, heat is generated from the ceramic block 1 and is radiated or released to the surrounding atmosphere. During the supply of the voltage to the ceramic block 1 and when the air is not flowing through the through-holes 2, the temperature of the block 1 rises by the greatest amount at the center portion between the electrodes 3 and 4, and by a gradually decreasing amount towards the opposite surfaces provided with the electrodes 3 and 4. This temperature distribution along the direction of thickness of the ceramic block 1 is shown by a curve W0 in FIG. 2. When the fan 5 is driven to generate wind W in the direction shown by the arrows of FIG. 2, the heat in the ceramic block 1, particularly in an intake region A-B close to the surface of the block which confronts the coming air, is released to the air for heating the air passing through the through-holes 2. As a consequence, the temperature in the intake region A-B is reduced, thus shifting the temperature peak towards an outlet region B-C located close to the other surface of the block 1. Therefore, the temperature distribution along the direction of thickness of the ceramic block 1 under the above condition results in a curve W1 shown in FIG. 2.
Since the material constituting the block 1 has a positive temperature coefficient characteristic, its resistance increases with increased temperature. Therefore, when the temperature distribution along the thickness direction of the ceramic block 1 corresponds to the curve W1, the resistance in the outlet region B-C becomes considerably higher than the resistance in the intake region A-B. Since the direction of electric current flow through the ceramic block 1 is in alignment with the thickness direction of the block 1, i.e., the direction of air flow, the electric resistance change in the outlet region B-C strongly influences the amount of electric current flow through the intake region A-B. Accordingly, the conventional heating device has such a disadvantage that the electric current flowing through the block 1 between A and C (FIG. 2) is undesirably limited by the high resistance in the outlet region B-C, causing a so-called pinch effect. Therefore, the heat generation is effected more efficiently in the outlet region B-C than in the intake region A-B where the heat release from the block 1 to the air is effected eminently. Thus, as a whole, the conventional heating device heat radiation efficiency.
Furthermore, since the ceramic block 1 directly touches, and releases the heat to, the incoming air, the conventional heating device has the additional disadvantage that the heat generated from the ceramic block 1 may become unstable particularly when the wind velocity increases abruptly, as explained below.
Generally, when the wind velocity increases, more heat is released from the ceramic block 1 to the passing air, causing a temperature drop in the ceramic block 1. This temperature drop results in the decrease of the resistance of the block 1. Thus, the current flowing through the block 1 increases to enhance the heat generation. However, if the wind velocity is increased abruptly as often caused by the change in the speed of the fan 5, the temperature drop in the ceramic block 1 is instantaneously dropped to instantaneously decrease the resistance of the block 1, causing a rapid increase of the current flowing through the block 1. This rapid increase of the current enhances the heat generation to rise the temperature of the block 1 above the temperature at which the ceramic block 1 loses its positive temperature coefficient characteristics (i.e. exhibits a negative temperature coefficient characteristic) and, as a result, the resistance of the block 1 becomes unstable. Thus, the power consumed in the ceramic block 1 may be undesirably oscillated causing an undesirable fluctuation in temperature.